FIG. 1A is a simplified circuit diagram of a traditional charger based on a pulse width modulation (PWM) control fly-back AC/DC converter. A transformer TX1 transfers energy received from a primary side source to a secondary side to power a load. The transformer TX1 has a first end of its primary coil connected to an input voltage VBulk, presumably a rectification output from an AC wall outlet. A second end of the transformer primary coil is connected to a main switch Q1 to regulate a current through the primary coil of the transformer for energy to be transferred to the secondary side of the transformer. A main controller is located on the primary side of the transformer to control the on and off of the main switch. A feedback loop with an error amplifier located at the secondary side of the transformer provides the output information back to the controller on the primary side through an opto-coupler. As shown in FIG. 1B, an operation frequency of circuit in FIG. 1A is limited to 65 kHz to 85 kHz at peak load. The PWM controller has a control bandwidth (BW) limited by current mode control loop bandwidth (BW˜0.1×fs) around one tenth of the switching frequency. As a result of the low operation frequency and narrow control bandwidth, the output voltage transient response is slow. FIG. 1C shows the large fluctuations of output voltage, Vout, transient response when a load transits between no load and 100% load, due to slow transition of operation frequency fs. Furthermore, for a conventional PWM controller, in order to maintain high conversion efficiency corresponding to the load condition change, it is necessary to switch the operation of a PWM controller between different operation modes of Continue Conduction Mode (CCM) and Discontinue Conduction Mode (DCM). Constant current compensation loop and constant voltage compensation loop are usually required to maintain stable operation of the controller. Therefore, a traditional charger based on a PWM control fly-back AC/DC converter inevitably requires extra components.
FIG. 1D and FIG. 1E show respectively a top view and a cross sectional view of a conventional vertical MOSFET transistor commonly used in a traditional charger of FIG. 1A. The transistor has drain electrode D located on a bottom surface of a transistor die connected to a die paddle of a lead frame having a bottom surface exposed from an encapsulation. A source electrode and a gate electrode are located on a top surface of the transistor die. The source electrode and the gate electrode are connected to a source lead S and a gate lead G. FIG. 1F shows a PCB layout 10 for a traditional charger of FIG. 1A. The PCB layout 10 is configured to receive a conventional MOSFET device of FIG. 1D and FIG. 1E. The conventional MOSFET device has a small area source lead connected to a small copper pad 11 on the PCB and a large area drain lead 14 connected to a large copper pad area 12 on the PCB. The drain electrode of the MOSFET chip is connected to the transformer TX1 through the large contact area between drain lead 14 and copper pad area 12. The source electrode of the MOSFET chip is connected to ground through resistor R2. The performance of the PCB layout 10 is not optimized due to unavoidable tradeoff between thermal dissipation and electromagnetic interference (EMI) noise reduction. The MOSFET device Q1 is hot and needs a large copper pad area 12 (for example, larger than 10 mm in length and 5 mm in width) for cooling. However, the large area drain lead 14 has high voltage and has high dv/dt value. It couples EMI noise to the system. This may not be a problem for low voltage applications. However, for high voltage applications such as 500V or higher, the EMI noise is high due to the fast changing and high drain voltage. It requires a small copper pad area 12 to reduce the EMI noise. This is in contrary to the need of having larger copper pad area 12 for cooling purpose. The tradeoff of a large copper pad area 12 is large EMI noise. In order to meet regulation requirements, additional bulky heat sink and metal shielding are used to improve thermal performance and RFI shielding. Furthermore, for high voltage applications, the high voltage drain lead with large area will demand large safety space therefore increasing the device area, making it challenging to minimize the device size while keeping safety space for high voltage.
Desirable features of a charger for a portable device include high performance to provide safe charging without damaging the device to be charged, fast charge to save time and compact size to save space for the convenience of mobility. Such desirable features would force a charger to use less component counts and smaller size components such as smaller transformers and smaller capacitors with the capability to handle higher power density without much cost increase. The increase of the power density would cause thermal and EMI problems. The use of smaller components or less component counts would potentially impact the performance of a charger. Therefore, the best charger, currently available in the market, provides less than 0.5 W/CC power density. The present disclosure provides solutions beyond 0.5 W/CC by applying novel system circuit control, co-packaging a main switch and a control integrated circuit (IC) on a single die paddle, and using a four-layer printed circuit board (PCB). Therefore, EMI is reduced, thermal performance is improved, and fast turn-on is achieved.